# Thermodynamics-why is this constant pressure?

1. Feb 20, 2012

### pyroknife

A piston-cylinder device contains 0.85kg of refrigerant at -10 deg C. The piston that is free to move has a mass of 12kg and diameter of 25cm. the local atmospheric pressure is 88kPa. Now, heat is transferred to the refrigerant-134a until the temperature is 15 deg C. Determine the final pressure.

The solution in the textbook says that the final pressure is equal to the initial pressure. I don't understand how this is an isobaric process.

2. Feb 20, 2012

### SammyS

Staff Emeritus
The piston is free to move.

3. Feb 21, 2012

### pyroknife

I don't understand why that means it's constant pressure. Are all pistons that are free to move considered constant pressure? If the piston is not free to move that means it's constant volume right?

4. Feb 21, 2012

### Greg-ulate

The piston is free to move and the atmospheric pressure on the outside is constant. If energy is transferred into the system as heat, the energy only has one place to go, into the kinetic energy of the gas, making it move faster. The more energetic gas particles push harder on the piston than air outside, so it moves. As the piston moves away particles that bounce off it lose momentum so they slow down, losing energy. The piston moves until it reaches the equilibrium point where the gas inside pushes just as hard as the gas outside.

Some of the heat energy has gone into moving the piston, some of the energy has gone into increasing the temperature of the gas. Work has been done to move the piston.

Any amount of gas will push on the walls of the container its in. If a piston is free to move, it requires a equal and opposite force to keep it where it is. Therefore, yes a piston that is free to move will move unless the pressure is equal on each side. Yes a piston that is not free to move will preserve the volume of the container.